The objective of this research proposal is to study some of the electrochemical properties of Purkinje fibers of sheep heart, which may provide new insights into the electrophysiological functions of the heart muscle cells. The major method of this research is the use of cation-selective glass microelectrodes and conventional micropipette electrodes to measure ion activities and membrane potentials. The intracellular K and Na ion activities will be measured from the heart muscle cells at different external K concentrations, and the K equilibrium potentials calculated from the K activities will be compared with the measured membrane potentials. Relative membrane permeabilities and conductances of K, Na, Cl, Li, Rb, and Cs ions as function of membrane potential will be determined by rapid changes of external concentration of the ions as well as on the basis of the ion activity measurements. This proposal is also aimed at elucidating the mechanisms of spontaneous activities (pacemaker activities) of the Purkinje fibers which can be induced by low external K concentrations. The electrophysiological roles of clefts between the cells will be sought to be clarified by the ion activity measurements and continuous measurements of membrane potentials during rapid changes of external K concentration, from normal K concentration to high and low K concentrations. This proposed research also intends to carry out studies on electrogenic Na-K pumping of the heart muscle cells by direct measurements of intracellular K ion activities from K-depleted and Na-enriched cells.